The present invention relates to a system and device for pumping molten metal and, in particular, a monolithic rotor that does not include separate bearing members.
A number of submersible pumps used to pump molten metal (referred to herein as molten metal pumps) are known in the art. For example, U.S. Pat. No. 2,948,524 to Sweeney et al., U.S. Pat. No. 4,169,584 to Mangalick, U.S. Pat. No. 5,203,681 to Cooper, and pending U.S. patent application Ser. No. 08/439,739 to Cooper, the disclosures of which are incorporated herein by reference, all disclose molten metal pumps. The term submersible means that when the pump is in use, its base is submerged in a bath of molten metal.
Three basic types of pumps for pumping molten metal, such as aluminum, are utilized, circulation pumps, transfer pumps and gas-release pumps. Circulation pumps are used to circulate the molten metal within a bath, thereby equalizing the temperature of the molten metal and creating a uniformly consistent alloy. Most often, as is known by those skilled in the art, circulation pumps are used in conjunction with a reverbatory furnace having an external well. The well is usually an extension of the charging well where scrap metal is charged (i.e., added).
Transfer pumps are generally used to transfer molten metal from the external well of the furnace to a different location such as a ladle or another furnace.
Gas-release pumps, such as gas-injection pumps, circulate the molten metal while adding a gas into the flow of molten metal in order to xe2x80x9cdemagxe2x80x9d or xe2x80x9cdegasxe2x80x9d the molten metal. In the purification of molten metals, particularly aluminum, it is frequently desired to remove dissolved gases such as hydrogen, or dissolved metals, such as magnesium. As is known by those skilled in the art, the removing of dissolved gas is known as xe2x80x9cdegassingxe2x80x9d while the removal of magnesium is known as xe2x80x9cdemagging.xe2x80x9d
All molten-metal pumps include a pump base that comprises a housing, also called a casing, a pump chamber, which is an open area formed within the housing, and a discharge, which is a channel or conduit communicating with the chamber and leading from the chamber to an outlet formed in the exterior of the casing. A rotor, also called an impeller, is mounted in the pump chamber and connected to a drive system, which is typically one or more vertical shafts that eventually connect to a motor. As the drive system turns the rotor, the rotor pushes molten metal out of the pump chamber, through the discharge, out of the outlet and into the molten metal bath.
A bearing member is added to the pump casing, which is preferably a ceramic ring attached to the bottom edge of the chamber. The inner perimeter of the ring forms a first bearing surface. A corresponding bearing member, which is a ceramic ring (sometimes referred to as a rotor ring), is attached to the rotor, and its outer perimeter forms a second bearing surface. The rotor is vertically aligned in the pump chamber so that the second bearing surface of the rotor aligns with the first bearing surface of the pump chamber. When the rotor turns, the first bearing surface keeps the second bearing surface centered, which in turn keeps the rotor centered in the pump chamber.
A problem encountered with this arrangement is that the ceramic ring attached to the rotor is fragile and often breaks. It breaks during operation of the pump because of impact against the bearing surface or because pieces of solid material, such as brick or dross present within the aluminum bath, become wedged between the bearing surface and the second bearing surface. The ceramic ring attached to the rotor also breaks during start up because of thermal expansion. In this respect, whenever a rotor including a rotor ring is placed in the pump, the ring is quickly heated from the ambient air temperature within the factory to the temperature of molten aluminum. The ring then expands and can crack. To alleviate cracking due to thermal expansion, the furnace operator may slowly heat the entire furnace to prevent thermal shock to the ring, but this results in downtime and lost production. Finally, the rings are easily damaged during shipping.
The present invention solves these and other problems by providing a bearing system, which includes a plurality of bearing pins or wedges (collectively referred to herein as bearing members, bearing pins or pins), that are less prone to fracture than a bearing ring. The geometry of each pin allows for thermal expansion without breaking. Generally, the present invention is a plurality of solid, heat-resistant (preferably refractory material) pins that attached to a molten-metal pump rotor. The perimeter of the rotor containing the pins is called a bearing perimeter. The surfaces of the pins that align with the first bearing surface of the pump casing collectively form a second bearing surface.
The material forming each bearing pin is harder than the material forming the rotor, so as to provide a wear-resistant bearing surface. Preferably, a system according to the invention will include a rotor having a plurality of bearing pins equally radially spaced about the rotor. In use, the rotor is mounted within the pump chamber of a molten metal pump so that the bearing pins form a second bearing surface that aligns with the first bearing surface provided in the pump casing.
In another aspect of the invention, a first bearing surface consists of a plug of heat resistant material formed in the base of the molten metal pump chamber and the second bearing surface is formed by a surface of a bore or recess formed in the bottom of the rotor. When the rotor is placed in the pump chamber it is seated on the plug, which is received in the bore or recess in the rotor base. This configuration not only centers the rotor, it vertically aligns the rotor in the pump chamber as well. Furthermore, this arrangement can be reversed, with a plug extending from the bottom of the rotor and forming a second bearing surface. A recess or bore is then formed in the base of the pump chamber. The plug is received in the recess and a surface of the recess forms the first bearing surface.
Also disclosed is a rotor especially designed to receive the bearing pins and a molten metal pump including a rotor with bearing pins.
Furthermore, disclosed herein is a monolithic rotor that does not include a separate bearing member attached to it. This monolithic rotor has a second bearing surface, but has no separate bearing member such as a pin, plug or ring. The advantage of such a monolithic rotor is reduced manufacturing costs because machining the rotor can be accomplished in a simple operation; no bores, grooves or recesses must be formed in the rotor to receive separate bearing member(s). In addition, no cementing of separate bearing members is required. Such a monolithic rotor is preferably formed of a single material, such as graphite.
A monolithic rotor as disclosed herein is preferably used in conjunction with a rigid rotor shaft to motor shaft coupling, which keeps the rotor centered in the pump chamber. Additionally, a monolithic rotor may be used with a monolithic pump casing, wherein the casing has a first bearing surface formed of, and preferably integral with, the same material forming the rest of the pump casing.